1. Field of the Invention
The present invention relates generally to the field of mobile communications, and more specifically to a control device enabling handover between mobile communication systems.
2. Description of the Related Art
The so-called IMT-2000 (International Mobile Telecommunications 2000) system is well known in the field of mobile communications, and is capable of meeting requirements of high-speed communications, for example, at 2 Mbps. There are two standardized transmission methods in IMT-2000 systems, one is IMT-2000/FDD (Frequency Division Duplex), and the other is IMT-2000/TDD (Time Division Duplex). FDD is capable of providing wide area services with symmetric traffic for downlink and uplink channels, and therefore is suited to data and speech transmission with lower data rate, such as 384 kbps. On the other hand, TDD is capable of dynamically varying the traffic of downlink and uplink channels, and is preferred for data transmission at higher data rates.
FIG. 1 shows a schematic view of a mobile communication system 100 including two sub-mobile communication systems each using one of the above two transmission methods. Specifically, the mobile communication system 100 shown in FIG. 1 includes an FDD base station 10 constituting an IMT-2000/FDD system, a TDD base station 20 constituting an IMT-2000/TDD system, and a mobile terminal 30 operable in either of the above systems.
The FDD base station 10 provides services in a service area 60 covering a certain geographical region, and is capable of transmission by the FDD transmission method in the area 60. Similarly, the TDD base station 20 provides services in a service area 70, and is capable of transmission by the TDD transmission method in the area 70. The FDD service area 60 and the TDD service area 70 are partially overlapped geographically, and in this overlapped area, communication by either of the above two modes is possible.
In FIG. 1, the symbol P1 indicates the overlapped common area of the service area 60 and the service area 70, symbol P2 indicates the area in the service area 60 other than the overlapped area P1, symbol P3 indicates the area in the service area 70 other than the overlapped area P1, and symbol P4 indicates the area outside of both the service area 60 and the service area 70. For example, if the mobile terminal 30 is situated in the area P2, it is capable of communication in the FDD mode only; if the mobile terminal 30 is located in the area P3, it is capable of communication in the TDD mode only; whereas, if the mobile terminal 30 is located in the area P1, it is capable of communication in both the FDD mode and the TDD mode. But, if the mobile terminal 30 is located in the area P4, that is, out of the service areas 60 and 70, it is incapable of communication in either mode.
Note that, for the sake of simplicity, only one FDD base station is shown in FIG. 1. In practice many FDD base stations are provided, and the corresponding areas of individual FDD base stations (called a cell or a sector) form the service area 60. Similarly, the service area 70 is formed by cells of many TDD base stations 20.
Each FDD base station 10 and TDD base station 20 is connected to a not-shown network through a control device and is controlled by the control device.
Consider the case in which the mobile terminal 30 is in the area P2 and is conducting data transmission with the FDD base station 10 at a relatively high data rate, and assume the mobile terminal 30 is moving to the area P1. In the area P1, as mentioned above, communications by both the FDD method and the TDD method are possible, and the TDD transmission method is advantageous to high-speed communication. Therefore, in the area P1, it is preferable for the mobile terminal 30 that is presently in the FDD mode to use the TDD resource to communicate if the TDD resource is available. To the contrary, if the mobile terminal 30 is transmitting at a lower data rate but in the TDD mode, when it is moved to the area P1, there is advantage to switching the mobile terminal 30 to the FDD mode. In other words, when the mobile terminal 30 is communicating by using one of the communication resources, if the terminal can be switched to the other mode when necessary, it is possible to realize a dual-mode mobile communication system superior in efficiency of resource utilization.
To realize a system like the mobile communication system 100, it has been proposed that the mobile terminal 30 operated in one sub-system constantly monitor a control signal from the other sub-system, and if the levels of the received signals are higher than a preset value, the mobile terminal 30 be switched to communication with the other sub-system. For example, the mobile terminal 30 is communicating with the FDD base station 10 in the area P2, while monitoring the levels of the received control signals from the TDD base station 20. Assume the levels of the received control signals are still lower than a preset value at the present position of the mobile terminal 30. If the mobile terminal 30 is moved to the area P1, the levels of the received control signals become higher than the preset value. If the mobile terminal 30 is transmitting data at a higher data rate, and if the TDD communication resource is available, the transmission mode of the mobile terminal 30 is switched to TDD mode.
In such a way, it is certain the mobile terminal 30 communicating by using one of the communication resources can be switched to the other system when necessary. If this method for system switching is adopted, however, transmitting and receiving devices for constantly monitoring the control signals from the other system are necessary in addition to those used for the communication conducted presently, and all these devices have to be operated in synchronization with each other. As a result, consumption of electric power increases greatly. In addition, the control signals have to be monitored constantly no matter whether the transmission mode of the mobile terminal needs to be switched or not, so, the electric power is wasted if system switching does not occur. To avoid installing two sets of transmitting and receiving devices, it is conceived to use only one set of transmitting and receiving devices, and this set of devices may be used alternatively temporarily. With such a configuration, however, in order to perform communication, the mobile terminal 30 has to precisely control timing of transmitting and receiving signals and the timing of receiving and measuring the control signals. But generally, timing control places a heavy workload on a mobile terminal. Accordingly, for a mobile terminal having to pay attention to power consumption, this method is not appropriate.
On the other hand, at the mobile terminal 30, the levels of the received control signals from a base station may vary largely because of the propagation path or the phase, and thereby, the levels of the received signals do not strictly depend on the distance to the base station. Therefore, sometimes system switching is actually not necessary or this switching is not actually possible even though the levels of the received signals are higher than the preset value. In this case, if such kind of switching is still carried out, this also leads to unnecessary power consumption.